Metal particles in the context of the present invention include nanoparticles and submicroparticles. Nanoparticles in the context of the present invention are defined as particles which are smaller than 100 nm at least in one dimension. Microparticles are considered to be particles which are between 1 μm and 1000 μm in size in all three dimensions. Submicroparticles are defined as particles which are larger than 100 nm in all three dimensions and which are smaller than 1 μm in at least one dimension. A sol or colloid is a dispersion of nano- or submicroparticles in a liquid.
Important criteria for the properties and fields of use of nanoscale and submicroscale metal particles include the particle morphology, the mean particle size, the particle size distribution, the stability of the dispersions in terms of colloid chemistry, and the processing properties of the particles.
Metal colloids can be characterized with regard to particular properties using their UV/Vis spectra. For instance, they exhibit a so-called plasmon peak, which originates from a collective oscillation of conduction electrons as a reaction to an oscillating external electromagnetic field. The shape and size of the plasmon peak can be characterized by the Em+100/Em ratio where Em corresponds to the absorbance maximum of a plasmon peak and Em+100 to the absorbance of the metal sol in the UV/Vis spectrum at the absorbance maximum plus 100 nm. For silver nanoparticles, it has become an established convention to use the E500/Em ratio, i.e. to form the ratio of the absorbance at 500 nm and at the peak maximum. This is valid since an absorbance maximum between 400 and 420 nm can be assumed for silver nanoparticles. The shape and size of the plasmon peak can then be used to draw conclusions about the particle size and the particle size distribution of the sample. In addition, the UV/Vis spectrum also changes when the sample agglomerates: the plasmon peak decreases in intensity and broadens.
The prior art discloses various processes for producing metallic nanoparticles. A known principle is the direct chemical reduction of dissolved metal ions in the liquid phase. The aim of many variants of this method is the production of dispersions, stable in terms of colloid chemistry, of metallic nanoparticles with narrow particle size distribution and defined surface properties. The different variants are characterized by the selection of the reactants, the reaction conditions and the reaction regime. The production of metallic nanoparticles by this principle is generally carried out as a batch process. However, it has not been possible to date to synthesize such dispersions with a metal particle content of 1 g/l or higher without needing to perform a subsequent concentration step.
In this context, the expression “stable in terms of colloid chemistry” means that the properties of the colloidal dispersion or of the colloids themselves do not change significantly during the customary storage times before application, for example no significant aggregation or flocculation of the colloid particles takes place.
One possible further route to the production of nanoscale metal particles is the synthesis of nanoscale metal oxide particles which are reduced in a subsequent step.
The synthesis of silver oxide nanoparticles and their conversion to metallic silver is discussed, for example, in EP 1 493 780 A1. This document discloses a conductive composition which is capable of providing a conductive dye with excellent flexibility and a high conductivity comparable to that of metallic silver, without high temperatures being required for film formation.
The conductive composition comprises a particulate silver compound and a binder, and optionally a reducing agent and a binder. Silver oxide, silver carbonate, silver acetate and the like are used as the particulate silver compound. Ethylene glycol, diethylene glycol, ethylene glycol diacetate and the like are used as reducing agents. A fine powder or a thermally curing resin, such as a polyvalent phenol compound, phenol resin, alkyd resin or polyester resin, or a thermoplastic resin such as styrene resin or polyethylene terephthalate having an average particle diameter of 20 nm to 5 μm, is used as the binder.
Moreover, the average particle diameter of the particulate silver compound is preferably 0.01 to 10 μm.
EP 1 493 780 A1, however, does not disclose how concentrated dispersions of silver nanoparticles can be prepared. Instead, the particulate silver compound is reduced at temperatures of more than 150° C. in the binder to silver particles which fuse with one another.
Methods for producing concentrated nanoscale metal oxide dispersions and the further use thereof in the production of nanoscale metal particles have thus not been disclosed to date. There therefore still exists in the prior art the need for a process for producing concentrated metal particle nanosols, for example from concentrated nanoscale metal oxide dispersions.
The present invention has for its object to overcome at least one of the disadvantages mentioned in the prior art. More particularly, it has for its object to provide a process for producing metal particle sols with a metal particle content of ≧1 g/l.